There is often more people at the sunday afternoon Paris Skate, as it's open from beginners/familly and it's a smooth 20 km treck in the street of Paris.http://www.rollers-coquillages.org/
From the organizers:
"29.000 in 99, in 2006 1.000 (winter) et 10.000 (june july)."
Should be one of the bigest, organized during the whole year (as the friday one).

I was going over the web site for the association; they even got cops on rollerblades to cover the skate! I'm definitely going to make sure my next trip to Paris coincides with one, I'm dying to attend!

I have been to 6 of these in the 1999-2001 time period. I can confirm that it did hit the 28,000 number stated above and was regularly in the 20,000 range. I think the smallest I saw on a Friday night Pari-Roller skate was 10,000. I'm surprised that (as stated above) the Sunday daytime skate is now larger. Back then it was ~5000 people, and the level of skateness was lower.

I landed here in searching to find out how well attended the Friday night skate is now. Does anyone know?